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Abstract

The structure of oceanic crust is surprisingly uniform, which suggests that crustal accretionary processes at mid-ocean ridges must be broadly similar, despite their different spreading rates and seafloor morphologies. Seismic studies have revealed the presence of sub-axial magma chambers at fast, slow and intermediate spreading ridges, but constraints on their shape and size are generally restricted to the fast spreading East Pacific Rise. The aim of this study is to compare the processes of crustal accretion at fast, slow and intermediate ridges by investigating the detailed crustal structure and magma chamber geometry of a magmatically active intermediate spreading ridge, the Valu Fa Ridge. A multidisciplinary geophysical experiment was conducted over the Central Valu Fa Ridge and its overlap with the Northern Valu Fa Ridge during R/V Maurice Swing Cruise EW9512, and wide-angle seismic data, recorded on a set of digital ocean bottom seismometers, were used to generate velocity-depth models on two across-axis, two along-axis and two axis-parallel profiles. These models were further constrained by modelling of the normal incidence seismic and gravity data and the resulting combined models of crustal structure were interpreted to reveal that a composite magma chamber exists beneath the Valu Fa Ridge crest. The magma chamber consists of a thin, narrow (1-1.5 km) melt lens, with an interconnected melt fraction, overlying a wider (-4 km) region of hot rock or low melt fraction. A reflection from the top of the melt lens is identified on both the normal incidence seismic and wide-angle seismic data and delay- time modelling indicates that velocities as high as 5.5 km s(^-1) are achieved -250 m below the top of the melt lens. The main body of the magma chamber corresponds to the region of hot rock below the melt lens and is delineated by anomalously low velocities, extending down through seismic layer 3 to within 1.5-2 km of the Moho. Moho reflections from beneath the overlapping spreading centre and a low on the mantle Bouguer anomaly map implies that this region is currently, or has recently been, the site of enhanced magmatism. This observation is contrary to popular models of ridge segmentation and melt delivery. The transition from pre-rift crust (both island arc and back-arc crust) to post-rift material, marked by considerable thinning of seismic layer 2, has also been uniquely identified in this study and describes the limit of VFR-generated crust. The size and temporal stability of magma chambers are largely dependent on their magma budget and the Valu Fa Ridge magma chamber model, developed as part of this study, may bridge the gap between the large, long-lived magma chambers identified at the East Pacific Rise and the more transitory magma chambers proposed at slow spreading ridges. Melt ascends as small isolated pockets through the main body of the magma chamber at the Valu Fa Ridge and resides in the melt lens until eruption. Seismic layer 2 is constructed solely from material 'erupted' from the melt lens, with the main body of the magma chamber cooling to form seismic layer 3. Convection currents, induced by large thermal gradients at the sides of the magma chamber, both accelerate the cooling process, thus limiting its size, and helps to generate the thick layered sequences as observed in ophiolite studies. The entire crust is emplaced within the axial region and a distinct Moho is formed at -0 Ma.